Extension and feature flags

This merge request proposes to store available EGL and GL extensions as flags in bitfields instead of booleans. It also adds the concept of feature flags ensuring the availability of a minimal OpenGL ES version and/or extensions at run-time. New flags availability functions egl_extensions_has(), gl_extensions_has() and gl_features_has() are introduced. This makes the gl_renderer structure slightly more compact and organised and makes it easier to check for the presence of an extension or a feature at run-time.

This also includes a few extension handling fixes discovered along the way.